Sub-cooled nucleate boiling regions in a reactor core are described as areas where

• A. Boiling occurs at surfaces even when the bulk coolant is below its boiling point
• B. Borated water concentrations are minimized
• C. Coolant is at or near saturation and no local boiling occurs
• D. Sub-cooled coolant has no nucleate boiling at the surface

Answer: (A)

Sub-cooled boiling occurs when the liquid in the bulk is below its saturation temperature for the given pressure, but the surface of a heated fuel rod or component is hot enough to raise the local temperature above the saturation point. Bubbles form at that surface (nucleate boiling) and depart into the subcooled liquid, providing efficient heat transfer even though the bulk coolant isn’t at its boiling point. That description—boiling at surfaces despite the bulk being below the boiling temperature—best captures what sub-cooled nucleate boiling regions are.

The other statements don’t fit this behavior. Borated water concentrations relate to reactivity, not boiling regions. If the coolant were at or near saturation and no local boiling occurred, that would not be sub-cooled boiling. And saying sub-cooled coolant has no nucleate boiling at the surface directly contradicts the defining feature of sub-cooled boiling.